The demographic performance of the capitulum weevil, Larinus latus, on Onopordum thistles in its native and introduced ranges.

It is a widely held premise amongst biological control practitioners that an agent released into a new range free from constraints such as specific natural enemies will perform better than in its native range. However, differences in performance between an agent in its introduced and native range have seldom been quantified to test this premise. The release of the capitulum weevil, Larinus latus, for the control of Onopordum spp. thistles in Australia has provided an opportunity to do so. In 1996/97, four years after establishment of the weevil, data were collected on population density, capitulum attack rates, oviposition rates, developmental success and patterns of oviposition. Data from this population, which is still expanding and not yet resource limited, were compared and contrasted with data from a similar study of this agent in its native range in Greece, where populations of L. latus were found to be relatively stable and were influenced by predation and competition. The data confirmed greater survival in the absence of predators, but also indicated that basic demographic parameters such as fecundity can also change when an agent is displaced into a new environment.